The present invention relates to an electron emission film, a field emission cold cathode device using the electron emission film, and methods of manufacturing the same.
A micro cold cathode device of field emission type has an emitter and a gate electrode (and/or an anode electrode). When a voltage is applied across the emitter and the electrode, the emitter emits electrons. Cold cathode devices of this type have advantages such as a high response speed, radiation resistance, heat resistance, and high power, and extensive studies have been made therefor. For example, as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 10-149778 (Jun. 2, 1998) (U.S. Ser. No. 08/931,417; Sep. 16, 1997) filed by some of the present inventors, a cold cathode device is expected as a high-power/high-voltage switching device.
From the viewpoint of electron emission at a low electric field and stable high power emission, carbon-based materials have received a great deal of attention as emitter materials. Conventionally, diamond, graphite, amorphous carbon, and the like have been proposed as carbon-based materials for an emitter.
Low-electric-field electron emission characteristics of several V/xcexcm or less have been reported.
For example, low-electric-field electron emission by an amorphous carbon film formed on an Si substrate by the cathode arc method is disclosed in APL 68 (18), p. 2529, (1996) by some of the present inventors (G. A. J. Amaratunga et al.). This reference also discloses that nitrogen-doped amorphous carbon lowers the electron emission threshold field. Such a low electric field is found not only in an nitrogen-doped amorphous carbon film (a-C:N), which is prepared by the cathode arc method but also in a hydrogenated amorphous carbon film (a-C:N:H) prepared by plasma CVD.
Jpn. Pat. Appln. KOKAI Publication No. 10-149760 (Jun. 2, 1998) (U.S. Ser. No. 08/933,039; Sep. 18, 1997) filed by Masayuki Nakamura discloses a cold cathode device that has carbon nanotubes or fullerenes on an emitter. Jpn. Pat. Appln. KOKAI Publication No. 10-112253 (Apr. 28, 1998) filed by Bernard Cole discloses a cold cathode device which uses a film with a graphite structure formed by the cathode arc method as an electron emission film.
It is an object of the present invention to provide an electron emission film having more excellent characteristics of, e.g., electron emission, mechanical strength, and fabrication, than those of the above prior arts, a field emission cold cathode device using the electron emission film, and methods of manufacturing the same.
According to a first aspect of the present invention, there is provided an electron emission film comprising:
a first portion which consists essentially of amorphous carbon and forms a matrix; and
a second portion having a crystal structure which consists essentially of a two-dimensional network of six-membered carbon rings that are dispersed in the matrix and partially project from the matrix,
wherein a weight ratio of the first portion to the second portion is about 50:50 to 5:95, and the first portion contains an impurity acting as a donor.
According to a second aspect of the present invention, in the film of the first aspect, the impurity is contained at a concentration of about 4xc3x9710xe2x88x927 to 10 atom %.
According to a third aspect of the present invention, in the film of the first or second aspect, the impurity is nitrogen.
According to a fourth aspect of the present invention, there is provided a method of manufacturing the electron emission film of the third aspect, using a film forming apparatus,
the film forming apparatus comprising
a vacuum chamber for accommodating a substrate to be processed,
a carbon electrode and a counter electrode which are placed in the vacuum chamber to oppose each other,
a power supply for applying an AC power having a low frequency between the electrodes, and
a supply port for supplying nitrogen to an area near the carbon electrode, and
the method comprising:
a preparation step of placing the substrate in the vacuum chamber and setting the vacuum chamber to a vacuum; and
a film forming step of supplying nitrogen from the supply port and applying the AC power between the electrodes, to generate arc discharge and sublimate carbon from the carbon electrode, thereby depositing the electron emission film on the substrate.
According to a fifth aspect of the present invention, in the method of the fourth aspect, the frequency of the AC power is set to be about 10 to 500 Hz, in the film forming step. In place of the AC power, a DC power may be used such that it is applied as pulses with the frequency described above.
According to a sixth aspect of the present invention, in the method of the fourth or fifth aspect, the vacuum chamber is evacuated from an exhaust port located to be closer to the substrate than the counter electrode, in the film forming step.
According to a seventh aspect of the present invention, in the method of any one of the fourth to sixth aspects, a pressure on the substrate is set to be about 1xc3x9710xe2x88x924 to 1xc3x9710xe2x88x921 mbar.
According to an eighth aspect of the present invention, in the method of any one of the fourth to seventh aspects further comprises, after the film forming step, a step of etching a surface of the electron emission film using an etchant for preferentially etching the first portion relative to the second portion.
According to a ninth aspect of the present invention, in the method of the eighth aspect, the etchant is a solution containing hydrofluoric acid.
According to a 10th aspect of the present invention, there is provided a field emission cold cathode device comprising:
a support substrate;
an emitter arranged on the support substrate, the emitter having an electron emission surface formed of an electron emission film; and
an extraction electrode for extracting electrons from the emitter,
wherein the electron emission film comprises
a first portion which consists essentially of amorphous carbon and forms a matrix, and
a second portion having a crystal structure which consists essentially of a two-dimensional network of six-membered carbon rings that are dispersed in the matrix and partially project from the matrix,
wherein a weight ratio of the first portion to the second portion is about 50:50 to 5:95, and the first portion contains an impurity acting as a donor.
According to a 11th aspect of the present invention, in the device of the 10th aspect, the impurity is contained at a concentration of about 4xc3x9710xe2x88x927 to 10 atom %.
According to a 12th aspect of the present invention, in the device of the 10th or 11th aspect, the extraction electrode is formed of a gate electrode supported by the support substrate via a gate insulating film and opposing the emitter, and the gate insulating film consists essentially of silicon oxide.
According to a 13th aspect of the present invention, there is provided a method of manufacturing the field emission cold cathode device of the 12th aspect, comprising the steps of:
forming a multilayered structure having the support substrate, the electron emission film, a silicon oxide film to be the gate insulating film, and a conductive film to be the gate electrode, stacked in this order;
partially removing the conductive film in correspondence with the emitter to expose a selected portion of the silicon oxide film; and
etching the selected portion of the silicon oxide film using an etchant to expose the electron emission surface of the emitter and simultaneously etching the electron emission surface using the etchant, the etchant preferentially etching the first portion relative to the second portion of the electron emission film.
According to a 14th aspect of the present invention, in the method of the 13th aspect, the etchant is a solution containing hydrofluoric acid.
In this specification, a fullerene or a fullerene-like structure is defined as follows.
As described in many references, a fullerene has a spherical or tubular structure having, as a shell, a two-dimensional network of five-, six-, and seven-membered carbon rings, which mainly contains six-membered carbon rings, i.e., a graphite sheet. FIG. 4A is a view showing a nanotube as one such fullerene. There is also a structure shown in FIGS. 4B or 4C in which a plurality of fullerenes form concentric circles or a helix. These structures are called superfullerenes. A fullerene-like structure means a microstructure of a crystal formed from fullerenes or superfullerenes, or sheets or walls as part of fullerenes or superfullerenes, i.e., a two-dimensional network of carbon atoms mainly containing six-membered carbon rings.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.